A new surface activation and wafer-to-wafer bonding process that uses atmospheric pressure plasma has been developed by SUSS MicroTec, eliminating the need for a vacuum chamber and associated process time.
The process, called nanoPREP, results in post-bond anneal temperatures below 350 degrees Celsius — particularly useful for creating SOI and strained-Si using direct wafer bonding. Typical fusion-bonding anneal temperatures for these materials are in the range of 1000-1100 degrees Celsius — a factor that limits the choice of materials.
Limiting the post-bond annealing temperature to below 350 degrees Celsius is also useful for direct-bond applications in MEMS and MOEMS devices where integration into wafer-level packaging is used.
“Low-temperature annealing is also important for developments of materials integration, including the use of wafer bonding for III-V semiconductor hetero-integration and the integration of III-V semiconductors with Si,” notes SUSS MicroTec CEO Franz Richter. He also points out that plasma surface treatment might eliminate the need for wet chemistry treatments for direct-bond applications in some cases.
According to Richter, there are three active elements to the process: removal of absorbed contaminants, creation of appropriate end caps to the surface molecules, and the creation of reactive sites. “The plasma changes the surface by modifying the final bond group,” explains Richter. “An example is creating a surface rich in (-OH) on a Si wafer. The plasma may leave a higher level of dangling or unreacted atoms which, under the proper conditions, allow easy reaction with the target wafer.” — Debra Vogler, Senior Technical Editor